Straddle tools to inflate external casing packers with cement are known and described in U.S. Pat. No. 5,082,062, U.S. Pat. No. 5,186,258, U.S. Pat. No. 5,366,019 and U.S. Pat. No. 5,692,564. Other external casing barriers have also been used such as swelling packers or hydraulically or mechanically compressed packers. These various packer designs present their own series of complications when multiple barriers need to be placed at spaced locations. The swell packers take a long time to reach sealing position while the mechanically set packers are of limited use in deviated boreholes. The hydraulically set packers require balls to be delivered to seat and need ball catchers for the balls once pushed through the seats. The ball seats may also need to be blown out to avoid reducing the size of the production flow passage once production begins.
More recently cement formulations have advanced to the point where commercially available cements can be pumped in a borehole and hold the position where the cement is placed despite the presence of borehole fluids.
The present invention allows for single trip placement of cement rings at predetermined locations on a tubular string where access to the open hole is provided. A straddle tool allows isolation of these access openings in sequence so that cement can be delivered into a surrounding annular space without need to perforate and squeeze cement into the annular gap and the surrounding formation. The cement can be spotted in the tool and once straddling access valves in the string cement can be sequentially delivered preferably in a single trip moving up the hole to define as many zones as needed for the completion before production starts. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined by the appended claims.